This invention relates to polymer coating materials for optical fibers.
Optical fiber production generally involves drawing a glass fiber from a glass preform, which usually is of silica glass, then applying coating material to the fiber. The coating is applied instantly after draw to prevent contamination or contact of any kind with the nascent fiber surface. The coating material is typically a UV curable polymer. Dual coated optical fibers are usually coated with a first layer of relatively soft polymer and a second layer of a higher modulus polymer for maintaining high strength and abrasion resistance. The combination of a soft primary coating and a relatively rigid secondary coating contributes to minimizing microbending losses. Microbending losses occur when the fiber is subject to small radii deformations which typically result from kinks or dents in the fiber coating. These occur in the use environment when the fiber passes over sharp edges, or when particulate contaminants deform the surface of the fiber. Extensive work has been devoted to minimizing microbending losses. Most of these efforts have been toward improved engineering of the optical fiber coating. The mechanical properties of the coating strongly affect the resistance of the optical fiber to microbending losses.
Optical fibers are usually coated by a wet coating process, typically involving passing the newly drawn fiber through a reservoir of liquid prepolymer material, and then curing the prepolymer by exposure to curing radiation, most commonly, ultra-violet light. In a dual coating process, coatings are applied in tandem or simultaneously (using a two compartment and dual die applicator). The tandem arrangement applies a first coating layer which is cured, and the second coating layer is applied over the first, and cured. In the simultaneous dual coating arrangement, both coatings are applied in a prepolymer state, after which they are cured.
The usual fiber coating materials are UV cured polyacrylates. These polymers are sufficiently transparent to UV curing radiation, i.e., wavelengths typically in the range 200-400 nm, to allow full curing at high draw speeds. Other transparent coating materials, such as alkyl-substituted silicones and silsesquioxanes, aliphatic polyacrylates, polymethacrylates and vinyl ethers have also been used as UV cured optical fiber coatings. See e.g. S. A. Shama, E. S. Poklacki, J. M. Zimmerman xe2x80x9cUltraviolet-curable cationic vinyl ether polyurethane coating compositionsxe2x80x9d U.S. Pat. No. 4,956,198 (1990); S. C. Lapin, A. C. Levy xe2x80x9cVinyl ether based optical fiber coatingsxe2x80x9d U.S. Pat. No. 5,139,872 (1992);P. J. Shustack xe2x80x9cUltraviolet radiation-curable coatings for optical fibersxe2x80x9d U.S. Pat. No. 5,352,712 (1994). The coating technology using UV curable materials is well developed. Coatings using visible light for curing, i.e. light below 600 nm, may also be used.
To increase the resistance of the fiber coating too abrasion damage, various fillers have been added to the polymer coating. Hard particulate materials, for example, titania, alumina or silica, would appear to be good candidates for such fillers. Fumed silica is suggested as a filler in, for example, U.S. Pat. No. 5,188,864, issued Feb. 23, 1993. However, there are a variety of known reasons why particulate fillers in-optical fiber coatings should be avoided. These include reduction in fiber test strength, and impairment of the UV curing process due to the opacity of particulate fillers.
An optical fiber coating parameter that is important in minimizing microbending losses is the bending modulus or bend strength of the coated fiber. Any coating modification that adds bend strength to the finished fiber would appear to reduce the potential for microbending losses. At the same time however, it is important that the fiber coating not be too stiff. Excessive stiffness in the overall coating leads to strains in the fiber from normal bending.
Optical fiber coatings with enhanced microbending resistance are continually being sought in fiber coating technology.
We have discovered new filler materials for optical fiber coatings that increase the microbend strength of the fiber without the adverse effects just mentioned. The new fillers are one or more of a class of so-called nanoclay particulates. These particulates comprise naturally occurring clay particles with a plate-like morphology, and very small overall dimensions. The clay platelets, when aligned along the axis of the optical fiber, increase significantly the microbending modulus of the coating. With proper choice of the particle dimensions, the efficiency of the UV curing process is essentially unaffected.